Field of the Invention
This invention relates to high strength, high toughness steel alloys, and in particular, to such an alloy that can be tempered at a significantly higher temperature without significant loss of tensile strength. The invention also relates to a high strength, high toughness, tempered steel article.
Description of the Related Art
Age-hardenable martensitic steels that provide a combination of very high strength and fracture toughness are known. Among the known steels are those described in U.S. Pat. No. 4,076,525 and U.S. Pat. No. 5,087,415. The former is known as AF1410 alloy and the latter is sold under the registered trademark AERMET. The combination of very high strength and toughness provided by those alloys is a result of their compositions which include significant amounts of nickel, cobalt, and molybdenum, elements that are typically among the most expensive alloying elements available. Consequently, those steels are sold at a significant premium compared to other alloys that do not contain such elements.
More recently, a steel alloy has been developed that provides a combination of high strength and high toughness without the need for alloying additions such as cobalt and molybdenum. One such steel is described in U.S. Pat. No. 7,067,019. The steel described in that patent is an air hardening CuNiCr steel that excludes cobalt and molybdenum. In testing, the alloy described in the '019 patent has been shown to provide a tensile strength of about 280 ksi together with a fracture toughness of about 90 ksi √in. The alloy is hardened and tempered to achieve that combination of strength and toughness. The tempering temperature is limited to not more than about 400° F. in order to avoid softening of the alloy and a corresponding loss of strength.
The alloy described in the '019 patent is not a stainless steel and therefore, it must be plated to resist corrosion. Material specifications for aerospace applications of the alloy require that the alloy be heated at 375° F. for at least 23 hours after being plated in order to remove hydrogen adsorbed during the plating process. Hydrogen must be removed because it leads to embrittlement of the alloy and adversely affects the toughness provided by the alloy. Because this alloy is tempered at 400° F., the 23 hour 375° F. post-plating heat treatment results in over-tempering of parts made from the alloy such that a tensile strength of at least 280 ksi cannot be provided. It would be desirable to have a CuNiCr alloy that can be hardened and tempered to provide a tensile strength of at least 280 ksi and a fracture toughness of about 90 ksi √in, and maintain that combination of strength and toughness when heated at about 375° F. for at least 23 hours, subsequent to being hardened and tempered.